Conventionally, a known transport vehicle includes a loading platform and a plurality of travel devices as shown by Japanese Laid-Open Patent Publication No. 11-246198. Cargos such as a plant, a bridge, and a rolled steel coil are loaded on the loading platform. The plurality of travel devices are arranged in a longitudinal direction and a width direction of the loading platform. The respective travel devices are independently steerable. Steering the respective travel devices allows the transport vehicle to travel in any directions of front to back and left to right directions.
By referring to FIG. 5A to FIG. 6B, a description will be given of travel devices 201 of a transport vehicle 200 disclosed in Japanese Laid-Open Patent Publication No. 11-246198. FIG. 5A is a front view of the transport vehicle 200 that includes the conventional travel devices 201 and travels in a straight line. FIG. 5B is a front view of the transport vehicle 200 that traverses. FIG. 6A is a side view of the travel device 201 in the case where a wheel 207 is moved in an upward direction. FIG. 6B is a side view of the travel device 201 in the case where the wheel 207 is moved in a downward direction. In FIG. 5A, for ease of understanding, a motor 209 is omitted.
As illustrated in FIG. 5A and FIG. 5B, the travel devices 201 are arranged side by side at a lower portion of a loading platform 200a, and are devices that allow traveling of the loading platform 200a. The travel device 201 mainly includes an arm 202, a swing arm 203, a spindle 204, an axle box 205, wheels 207, and a shock absorber (a hydraulic cylinder) 208. The arm 202 includes an upper end supported at the lower portion of the loading platform 200a turnably in the horizontal direction around the vertical axis. The swing arm 203 includes one end supported at a lower end of the arm 202 swingably in up and down directions around the horizontal axis. The spindle 204 is disposed to protrude from a distal end of the swing arm 203 in the horizontal direction. The axle box 205 is journaled swingably to the spindle 204. The wheels 207 are rotatably supported at both sides of an axle shaft 206 disposed at the axle box 205. The shock absorber 208 is coupled to the arm 202 and the swing arm 203, and absorbs movement of the swing arm 203 in the up and down directions. The motor 209 is mounted on the axle box 205. The motor 209 is driven to rotatably drive the wheels 207.
As illustrated in FIG. 5A, the transport vehicle 200 can travel in a straight line by steering the travel devices 201 such that a width direction (a right-left direction as illustrated in FIG. 5A of the loading platform 200a is aligned with an axial direction of the axle shaft 206 and by driving the motor 209 to rotate the wheels 207. As illustrated in FIG. 5B, the transport vehicle 200 can traverse by steering the travel devices 201 such that a longitudinal direction (a vertical direction of the paper in FIG. 5B of the loading platform 200a is aligned with an axial direction of the axle shaft 206 and by driving the motor 209 to rotate the wheels 207.
As illustrated in FIG. 6A and FIG. 6B, the swing arm 203 of the travel device 201 swings in up and down directions corresponding to unevenness of a road surface G where the transport vehicle 200 travels. A horizontal distance L1 between the lower end of the arm 202 and a ground center n of the wheel 207 varies significantly in the case where: the swing arm 203 swings in the upward direction and then the wheel 207 moves in the upward direction (see FIG. 6A, and the swing arm 203 swings in the downward direction and then the wheel 207 moves in the downward direction (see FIG. 6B). Simultaneously, a relative position of the ground center n of the wheel 207 with respect to a turning center line R of the arm 202 varies significantly. Up and down swinging of the swing arm 202 and up and down movement of the wheel 207 that are associated with these changes are absorbed by the shock absorber 208.